Ripley is a tiny REPL toolkit for Swift Playgrounds for iPad. It provides three public types:

• RipleyEngine - a protocol allowing you to power Ripley with an Eval of your implementation,
• RipleyTheme - a simple struct defining how Ripley should look like,
• RipleyViewController - a Terminal-style view controller you can use for Read and Print operations.

Usage

To start playing with Ripley, check out this repository on your iPad (e.g. via Working Copy) and use Swift Playgrounds "Locations" button to open the Ripley.playground via Files.app. Tap Run Code to build and run Ripley - everything you type, will be echo'd back to you.

To add Ripley to your own Playground:

1. open the Playground in Working Copy or an editor like GoCoEdit,
2. create a Sources directory inside the Playground,
3. copy Ripley.swift over.

There is not much other configuration necessary - simply add a RipleyTheme, adopt RipleyEngine on your REPL object, set an instance of RipleyViewController as your Playground's live view, and you are ready to go!

Single and multi-line sources

Ripley passes the source as-is to the eval method of your choosing. The trigger for eval is a new line character. (i.e. Return on your keyboard) In case you would like to type multi-line sources, escape your newline by appending the \ to the end of the line. Ripley will remove the backslashes from the resulting source for you.

For example:

> def {fun} (\ {args body} { \
  def (head args) (\ (tail args) body) \
})

Will be passed to the eval method as:

def {fun} (\ {args body} {
  def (head args) (\ (tail args) body)
})

Input history

You can use the up and down arrows on the hardware keyboard attached to your iPad to scroll through the input history. Input history is persisted in a Swift array on a per-runtime basis and can be modified by changing the inputHistory variable on the RipleyViewController. This is particularly useful for pre-filling snippets that you might need often in your REPL.

Background

I decided to travel for my most recent vacation with just an iPad. It so happened, that during my trip Bali was both celebrating Nyepi and having three days of constant rain. Stuck in a hotel bar, I started working through Build Your Own Lisp and Crafting Interpreters. Keeping the little Lisp apps as strings got old really quick, so Ripley is one of the outcomes.

Enjoy!

I had a lot of fun implementing Ripley. While BaLISP - the Lisp I implemented while vacationing - is not quite ready for public consumption yet, Ripley is pretty much there. If you enjoy using it, let me know on Twitter!